A motor-driven brake system for rail vehicles and the like can comprise a brake mechanism having jaws or shoes which confront and engage a movable brake member such as a disk, and an actuator connected by a rod to one or both of the brake shoes.
In the system described in German patent document DE-PS No. 30 10 335, an electric motor is operated to drive a threaded spindle which cooperates with a nut on the actuator to retract the actuator and thereby displace the brake into its open, disengaged or relieved position while, at the same time, compressing a force-storing spring which acts upon the actuator when the motor is deenergized and the spindle is released to drive the actuator in the brake-engaging stroke.
The brake actuator can be a shiftable tube having an eye connected to the brake rod and hence to the brake shoe and the force-storing spring, of course, will then generate a braking force which is transmitted to the brake shoe through the mechanical linkage described.
The electric motor is provided with a control circuit which can feed the electric motor with a voltage variable between the minimum value and a maximum value to operate the brake force.
The control circuit, in turn, receives a signal from the brake mechanism via a signal transmitter or sensor which can be in the form of a Wheatstone bridge having two variable resistances.
The brake system thus described, therefore, allows the electric motor, by appropriate electrical energization thereof, during expansion of the force-storage spring, to control the brake force and provide an especially effective sensing and control of the braking action applied by the brake shoes to the rotary member or disk of the brake mechanism.
The control of the electric voltage for the storage-spring stressing motor is effected through a Wheatstone-bridge arrangement which in a zero balance state (corresponding to complete equality of the two variable resistances and full balance of the bridge) brings the motor to standstill and turns on a holding brake which retains the threaded spindle against rotation.
In practice, it has been found to be necessary with this type of brake system to subject the brake device, comprised of the motor and the force-storing spring, to calibration on a testing stand. From the spring characteristic, it is possible to determine the brake force delivered to the brake mechanism for any particular stroke of the actuator.
It has already been indicated that the setting of the bridge to full balance or zero output, corresponding to cutoff of the motor and actuation of the holding brake for the spindle depends upon the development of a balance in two branches of the bridge. To enable this to occur for a variety of positions of the actuator and hence in accordance with the spring characteristic, it has been necessary heretofore to connect a branch of the bridge to the actuator so that the bridge also was capable of sensing the position of the actuator.
The drawback of the bridge, of course, was that calibration of the brake device was necessary on a test stand in a series of tests. In spite of such calibrations, however, it was impossible to avoid a spread or deviation from the calibration values. Furthermore, different aging conditions affected the spring characteristics of different brake devices differently and over the life of the brake units, considerable change was noted from the calibrated or standardized determinations originally made. Naturally removal of the unit for calibration and standardization posed a significant hardship.
Mention should be made of German patent document DE-OS No. 22 08 936 which describes a setpoint/actual value comparison in association with a Wheatstone bridge and in conjunction with brakes for other purposes and of other constructions, especially hydraulic brakes. In this arrangement, a barometer-like sensor is provided with a wear-sensitive rod together with a wiper resistor within the Wheatstone bridge to permit after-adjustment during operation of the system.
Mention may also be made of the United Kingdom Pat. No. 1,352,869 which discloses the use of electrical photoresistors which are operated in response to an exciting-light current controlled by the mechanical swing of a braking process at a trailer drawn by a motor vehicle.